1. Field of the Invention
The present invention relates to the field of gamma cameras and, more particularly, to the field of gamma cameras comprising two detector heads capable of carrying out tomographic examinations.
A gamma camera is an apparatus comprising a base that is fixed or movable with respect to the ground and having at least one arm provided, at its free end, with a detector. This detector has an array of photomultiplier tubes whose input faces, which are juxtaposed with one another, cover the detection surface of the detector head and determine its field of detection.
2. Description of the Prior Art
The principle of the examination is as follows. A radioactive substance is injected into a patient to be examined. The gamma radiation produced by the radioactive emission that emanates from this patient goes through a collimator and excites a scintillator crystal of the detector which converts the energy of the gamma photons into a light energy that is detectable by the photomultiplier tubes. These photomultipliers then produce electrical signals depending on the light intensity received.
By carrying out operations of barycentric localization on all these electrical signals it is possible, in a known way, to determine the X, Y coordinates of the scintillation in the field of detection. An incremental acquisition is then made by totalizing the number of scintillations detected throughout the field of detection.
Thus, by leaving the detector head in a given position for a certain time above the body examined, it is possible, for a given viewing angle, to obtain an image that reveals the concentration of radioactive substance in the body.
It has become the common practice to use a rotating base. It is possible then to make tomographic examinations that consist in acquiring one image per viewing angle for a large number of viewing angles, evenly spaced out over an angular sector of at least 180.degree.. The image of an examined volume is then reconstituted with computation algorithms.
In order to improve the sensitivity, gamma cameras with rotating bases having two detector heads instead of only one have been brought into use, these detector heads being positioned before each other, either face to face or, preferably at a fixed angle, preferably equal to 90.degree. with respect to each other Furthermore, a device commonly called a hoist or elevator has been used, enabling the arms to move away from each other or to approach each other so as to bring the detector heads as close as possible to the patient's body.
FIG. 1 illustrates a gamma camera comprising the above-mentioned different improvements. It is provided with two arms, fixed horizontally to a vertical base 1 that is itself fixed to a frame 2. For obvious reasons of symmetry, only the upper arm, referenced 3 in figure 1, shall be described. This arm 3 holds a detector head 4 at its free end, by means of a U-shaped stirrup 5.
The base 1 rotates about an axis A1 which is conventionally the rotational axis of the gamma camera.
Each detector head 4 is fixed to the stirrup 5 rotationally about an axis A2 passing substantially through the center of said head and perpendicularly to the axis A1. The axis A2 is called, by convention, the axis of orientation of the detector head.
Furthermore, each of these detector heads 4 is fixed rotationally about an axis A3 that is perpendicular to the axis A2 and passes through two bearings fixing the detector head 4 to the stirrup 5. The axis A3 is called, by convention, the axis of "angulation" of the detector head.
Furthermore, each arm 3 can carry out a radial translational motion along the rotating base in a radial translational direction R so as to approach or move away from the axis of rotation A1, within the limits of a window 6. The translational motions of the two arms are preferably symmetrical. The windows 6 are furthermore closed by the sliding of a system of telescopic curtains.
Hence, many positions of the detector heads 4 are possible. Each position is defined by a set of coordinates (r, .alpha.1, .alpha.2, .alpha.3) where r is a radial translation of the arms along R, .alpha.1 is an angle of rotation of the gamma camera along A1, .alpha.2 is an angle of orientation of the detector head considered along A2 and .alpha.3 is an angle of angulation of this very same head along A3.
The complex functions that the stirrup 5 has to perform need to be emphasized.
This stirrup 5 must first of all hold the detector head 4 in a totally reliable manner. It must then enable, firstly, the orientation of the detector heads 4 and, secondly, their angulation.
In order to carry out these different functions, the arm 3 supports the stirrup 5 at a single point, located substantially at the center of the horizontal part of said stirrup 5. Furthermore, the vertical uprights of the U forming the stirrup 5 have a height that is great enough to enable the obtaining of an angulation of the detector heads 4 by an angle .alpha.3 at least equal to 90.degree. without the edges of said heads 4 coming into contact with the above-mentioned horizontal part.
Furthermore, because of the height of the vertical uprights of the stirrup 5, the arm 3 is curved so that its end fixed to the base 1 is close enough to the axis of rotation A1 of the gamma camera. Nevertheless, the arm 3 is then longer, and it needs to be solidly joined to the base 1.
The present invention is aimed at proposing a gamma camera that overcomes the above-mentioned drawbacks and makes it possible, in particular, to obtain a structure that is simplified but capable of bearing a greater, hence heavier, head, eliminating the stirrup of the prior art without thereby in any way eliminating the above-mentioned different functions of translation and rotation, this camera being particularly reliable and also very attractive as regards cost.